1. Field of the Invention
The present invention relates to a pushbutton switch key sheet for use in an operating portion of various apparatuses, such as a mobile phone, a personal digital assistant (PDA), a car navigation apparatus, or a car audio apparatus, and more particularly to a key sheet having keytops on a rigid base sheet.
2. Description of the Related Art
As a pushbutton switch key sheet for use in an operating portion of various apparatuses, there is widely used one in which keytops are attached by adhesive to a base sheet formed of a rubber-like elastic material (See JP 11-144549 A). Recently, however, from a viewpoint of outward appearance and diversity in design, there has been increasing a demand for a key sheet using a base sheet using as the base a hard resin plate instead of a base sheet formed of a rubber-like elastic material (See JP 2003-178639 A).
In such a key sheet, a base sheet is formed such that a plurality of through-holes are formed in a thin hard resin plate, and each of the through-holes is covered with a thin-walled operating portion consisting of a rubber-like elastic material. A keytop is fixed to each operating portion by an adhesive or the like, thus a key sheet is formed (See JP 2003-178639 A, the first embodiment (paragraph 0014, FIG. 1)).
This conventional key sheet, however, has a problem in that its molding takes effort, resulting in a rather high cost. That is, to manufacture the base sheet, it is necessary to form in the mold an inlet for the rubber-like elastic material corresponding to each through-hole of the hard resin plate. After molding, it is also necessary to perform gate cutting for each through-hole. This leads to a rather complicated mold structure, resulting in high cost and an increase in operational burden after molding.
Further, when the operating portions are formed by injection molding using a rubber-like elastic material such as a thermoplastic elastomer, there are formed relatively large protrusion-like molding marks on the front or the back surfaces of the operating portions. That is, when the molding marks protrude on the front surfaces of the operating portions, the keytops, which are attached by adhesive or the like, are attached obliquely, thereby impairing the outward appearance of the key sheet. When the molding marks protrude on the back surfaces of the operating portions, the contact switch of the printed circuit board is turned on/off by the molding marks at the time of depression of the keytops, so there is a fear of accurate depressing operation being hindered. Further, in the case of an illumination type key sheet, the existence of the molding marks in the operating portions leads to unevenness in illumination of the keytops, so there is also a fear of the outward appearance of the key sheet when the illumination as been impaired.
As means for solving the above-mentioned problems, there has been proposed a key sheet in which a rubber-like elastic material film is formed so as to cover the entire front or back surface of a hard resin plate while filling each through-hole (See JP 2003-178639 A, second embodiment (paragraphs 0016, 0017, FIG. 2), and third embodiment (paragraphs 0018, 0019, FIG. 3)).
In this conventional key sheet, however, the rubber-like elastic material film covers one surface of the hard resin plate, which means that the thickness of the rubber-like elastic material film is added to the thickness of the hard resin plate, with the result that the thickness of the base sheet as a whole increases. In particular, this conventional key sheet is not applicable to an apparatus of which a reduction in thickness on the order of millimeters or less is required, as in the case of an electronic device.